1. Field of the Invention
This invention relates to an improved light directing structure for use with a liquid crystal display and to a multiple function digital meter with improved ease of operation which incorporates the light directing structure. More particularly, it relates to such a light directing structure for increasing the contrast of a liquid crystal display and to a digital panel meter which is less prone to improper operation during resetting.
2. Description of the Prior Art
Aircraft instrumentation panels provide a very demanding environment for digital panel meter display technology. The displays of such panel meters must be readily visible under a wide variety of cockpit lighting conditions. Thus, even though light emitting diode (LED) displays are readily visible under ordinary lighting conditions, they are not readable in full sunlight, which is often encountered in an aircraft cockpit. On the other hand, liquid crystal displays are readily visible in full sunlight, but present visibility problems in a darkened cockpit. Backlighting for such liquid crystal displays has been provided in the prior art, but has hitherto not provided sufficient and uniform illumination to meet the demands of the aircraft cockpit environment.
There are a variety of digital panel meters presently available which are suitable for aircraft applications. For example, such meters available from Davtron, Incorporated, Redwood City, Calif., under the designations of Model 811B, Model 701B, and Model 655, respectively provide a multiple function clock, a digital ADF indicator, and a meter indicating five different aircraft operating parameters. These digital panel meters utilize incandescent illuminated digits in order to provide a high contrast display which is easily visible under a wide variety of cockpit lighting conditions. Gas discharge displays are also suitable for aircraft digital panel meters. However, both incandescent and gas discharge displays are expensive and have relatively large power demands. Gas discharge displays also have relatively short operating lifetimes, on the order of 2,000 hours, as compared to 30,000 hours for a liquid crystal display. Despite these disadvantages, the precision of digital displays has resulted in their widespread use in aircraft.
Also, prior art digital meters have been prone to improper operation, particularly to inadvertent resetting by those unfamiliar with the digital meter controls. Such inadvertent resetting can present a significant safety hazard in an aircraft by destroying information needed by the pilot.